1. Field of the Invention
This invention relates to shaft seals for preventing passage of contaminants. More particularly, the invention pertains to a seal that prevents the occurrence of underpressure otherwise tending to cause excessive wear of a sealing surface.
2. Description of the Prior Art
A radial shaft sealing ring having a dust protection lip is described in German Pat. No. 32 36 636. The sealing ring described in this patent connects the interior of a casing to a space bounded by two sealing lips and a shaft extending through an opening into a casing. A pressure-sensitive diaphragm opens whenever an overpressure is formed in the interior of the casing. The diaphragm opens toward the space between the sealing lips and shaft so that oil, in liquid or vapor form, within the casing passes through the open diaphragm onto the shaft.
The shaft sealing ring can be fitted only above the normal level of the lubricating oil within the casing; otherwise, it would leak continually. However, even if the sealing ring is located above the level of lubricating oil, oil spray within the gearbox casing enters the space between the sealing lips. Oil located within this space eventually wets the outer dust protection lip, whose purpose is to prevent entry of dust from outside the casing. A dust protection lip wetted with lubricating oil retains dust on the wetted surface, which abrades the shaft surface tending to remove the seal contrary to its intended purpose. A dry dust protection lip continually repels exterior dust.